1. Field of the Invention
The present invention relates generally to capacitor fabrication and more particularly to resin-filled AC capacitors.
2. Background of the Invention
High performance capacitors are often fabricated using a resin to cover capacitor elements. Typical capacitor designs employ a metal or plastic housing containing a chamber or chambers in which capacitor electrodes are inserted. FIG. 1 illustrates a high performance capacitor 100 of known structure. Capacitor 100 includes capacitor element (also hereinafter termed “section”) 102, capacitor case 104, capacitor terminals 106, and cover 108. During assembly of capacitor 100, capacitor section 102 is attached to cover 108. Section 102 is subsequently inserted into case 104 by placing cover 108 over base 105 such that cover 108 comes to rest on a top portion of case 104. Insulating fluid (not shown) is provided within case 104 to improve capacitor performance. By encapsulating section 102, insulating fluid serves to reduce gas voids to increase voltage for corona inception, limit water vapor and oxygen attack on section 102, and scavenge any activated gas molecules generated in the vicinity of section 102. These features are especially important for operation voltages of AC capacitors above about 300 V.
Because optimal capacitor performance is dependent on each capacitor section being completely covered by fluid, it is important that sufficient fluid be dispensed within a capacitor case. Known methods for dispensing fluid include performing a first fluid dispense, visually inspecting the capacitor with cover in place to see how much additional fluid is required to completely coat the capacitor elements, and performing a second fluid dispense to provide sufficient additional fluid to completely cover the capacitor elements. FIG. 1A illustrates a capacitor 120 after dispensing of fluid 122 and placement of cap 124 on base portion 126. Upper portion 127 of capacitor section 128 remains clearly uncovered by insulating fluid, indicating that a second fluid dispense is desirable. However, the above procedure is tedious and requires more sensing and dispensing equipment and steps than ideal.
Alternatively, a precise volume of resin needed to coat capacitor elements can be determined and used to dispense the appropriate resin volume in a single dispense. However the latter approach requires that an exact volume of the capacitor case and capacitor section be determined. This typically requires special winding equipment or a closed loop feedback system that is built into a dispensing unit.
Another approach to ensuring that a proper amount of resin is dispensed is to perform visual monitoring during the dispense process. This results in a much slower dispense process and may nevertheless result in air bubbles generated in the dispensed resin, as well as requiring additional equipment to operate.
In light of the foregoing, it will be apparent that there is a need to improve fabrication processes for capacitors having encapsulated capacitor sections.